Our objectives are: 1) to learn how the normal human intestine functions with respect to water, electrolyte and divalent cation transport, and 2) to determine how normal transport processes are deranged in diarrheal diseases and in diseases where malabsorption or hyperabsorption causes pathologic changes in the body. To reach this goal, we have developed accurate in vivo methods for studying intestinal absorption, secretion and permeability in normal subjects and in patients with intestinal and metabolic diseases. We have carried out detailed studies to elucidate normal transport processes in the jejunum and ileum and have developed conceptual models which we believe accurately depict the mechanism of water, sodium, potassium, hydrogen, chloride and bicarbonate transfer. We have applied our methods and techniques to the study of patients with celiac sprue, congenital chloridorrhea, pancreatic cholera and chronic renal disease. Current and planned research includes: 1) studies in patients with intractable diarrhea, 2) development of intravenous calcitonin as a model for studying small intestinal secretion in humans, 3) experiments on the design of optimum oral test solutions in the therapy of diarrhea, 4) studies of calcium and magnesium absorption in normal subjects under varying physiologic conditions (age, diet, etc.), and in patients with metabolic diseases such as uremia, osteoporosis, idiopathic hypercalciuria, adrenal steroid induced bone disease, and 5) a careful study of small bowel absorption-secretion in elderly people.